Exhaust gas emissions control in a gasoline engine utilizing a catalyst is most efficient when the catalyst feedgas air-fuel ratio is near stoichiometry. During real world operation of the gasoline engine, excursions from stoichiometry may occur. Ceria is commonly added to a catalyst to act as a buffer for oxygen storage to help curb the breakthrough of emissions and increase the operating window about the stoichiometric air-fuel ratio. The stored oxygen may be maintained at a desired set point based on catalyst monitor sensors and/or physics-based catalyst models.
One approach to control and diagnose exhaust gas emissions utilizes a physics-based model to determine the level of stored oxygen in a catalyst which includes a plurality of partial differential equations in one or more dimensions with a plurality of exhaust gas species. Another approach utilizes an axially averaged physics-based zero dimensional model which includes a plurality of exhaust gas species which may be grouped into an oxidant group and a reductant group.
However, the inventors have recognized an issue with the above approaches. Determining the level of stored oxygen utilizing a model that includes a plurality of partial differential equations in one or more dimensions may be difficult to implement and may require more processing power than typically available in an engine controller. Further, utilizing a zero dimensional model may neglect parameters and may not accurately predict emissions during cold start due to the reduced order of the model.
Thus in one example, the above issue may be at least partly addressed by a method for an engine exhaust system. In one embodiment, the method comprises adjusting a fuel injection amount based on a fractional oxidation state of a catalyst, the fractional oxidation state based on reaction rates of exhaust gas species in a one-dimensional model averaged over space and time mass balance and energy balance equations for a fluid phase and a washcoat of the catalyst. The gradients in the transverse direction are accounted for in the internal and external mass transfer coefficients. This may improve computational time of the one dimensional model by grouping the chemical exhaust gas species into two or fewer groups which may include an oxidant group and a reductant group wherein a single value of diffusivity may be used.
For example, the fractional oxidation state may be determined based on the one-dimensional model derived from a detailed two-dimensional model. The model may track the evolution of two or fewer grouped exhaust chemical species through the catalyst. Further, the model also accounts for the diffusion within the washcoat where the reactions take place through the use of effective mass transfer concept. In this way, a simplified one-dimensional model may be used to predict both a total oxygen storage capacity and fractional oxidation state of the catalyst. These may be used in feedback control of the engine air-fuel ratio in order to maintain the fractional oxidation state of the catalyst at a desired amount. Further, catalyst degradation may be indicated if the catalyst activity or the total oxygen storage amount is below a threshold.
The present disclosure may offer several advantages. For example, processing resources devoted to the catalyst model may be reduced. Further, emissions control may be improved by maintaining the catalyst at a desired fractional oxidation state. In addition, emissions during cold start may be accurately predicted for real time fueling control. Another advantage of the present approach is that it offers a non-intrusive catalyst monitor for control and diagnostics, which is less dependent on sensor location and hence will be equally applicable to both partial and full volume catalyst systems.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.